Any reference in this specification to prior art disclosures is not to be construed as an admission that the respective disclosures are common general knowledge, in Australia or elsewhere.
Nanofibres can be made in a number of forms from various materials. Carbon nanotubes (CNTs) are one example and they occur as either a single walled tube (SWNT) or a multi-walled tube (MWNT). The structure of SWNTs is that of a one-dimensional graphene sheet that is coiled about an axis to form the nanotube. MWNTs consist of a number of SWNTs all formed around a common axis. The diameters of SWNTs are typically less than ˜1 nm, whereas the diameters of MWNTs may comprise many tens of tubes with final diameters of the order of 50 nm or more. Lengths are commonly in the order of tens of microns for SWNTs, up to several millimeters for MWNTs.
Carbon nanotubes, particularly of the single-walled variety, have a range of spectacular properties that are of great technological interest: including high elastic modulus (˜1 TPa) and high mechanical strength (˜30 GPa) (R. H. Baughman, A. A. Zakhidov, and W. A. de Heer, Science, 297, 787-792, 2002). A low volumetric density (˜1330 kg/m3) means that the specific properties are even more exceptional when compared with most other materials, e.g., the modulus and strength of SWNTs are ˜20 and ˜50 times that of high tensile steel. SWNTs also display excellent transport properties such as high electrical conductivity (10-30 kS/cm) and high thermal conductivity (˜2000 W m−1 K−1).
A significant problem for the practical application of CNTs has been the absence of a method to assemble the trillions of nanotubes into macro-sized items, such as fibres or objects. One approach has been to use fluids such as surfactants or polymers to assemble the CNTs into macro-structures, but there are several problems associated with ‘wet’ processing of this kind. Firstly, dispersing CNTs into the fluids causes significant breakage of the CNTs, inhibiting the properties of the composites. Another problem is that the viscosity of the fluid increases rapidly with the concentration of the CNTs, which limits ultimate concentrations to less than 10%. Finally, if the CNTs are filtered from the dispersion to produce a CNT paper, it is found that residual traces of the fluids remain on the nanotubes that significantly reduce transport of electrons or phonons.
In a surprising development, it was shown (M. Zhang, S. Fang, A. Zakhidov, S. B. Lee, A. Aliev, C. Williams, K. Atkinson, R. H. Baughman, Science, 309, 1215 (2005) and International patent application PCT/US2005/41031) that twist could be used to spin CNTs into a yarn in much the same way as for conventional fibres. This successfully overcame the problems of wet processing, being based on solid-state processing of nanotubes. The construction of these yarns required much higher twists compared with conventional yarns because of their much smaller diameters. The authors of the cited paper reported that yarns with diameters of about 1 μm had quite good tenacity and high electrical conductivity for twists of about 50 000 m−1. Given the fineness of the nanotube yarn, ˜1 μm, which is 100 times smaller than the equivalent worsted yarn, the high levels of twist ensure the same helix angle of the nanotubes that in turn ensure reasonable tensile properties. This method utilises MWNTs grown in forests with the important property that once the nanotubes on an outer face of the forest are withdrawn, the nanotubes in the next row are pulled with it. This process continues indefinitely through the ranks of nanotubes in the forest, ultimately creating a continuous web of nanotubes that has sufficient integrity to be used by itself or twisted into a yarn. The webs and yarns have excellent mechanical strength and electrical conductivity and can be used in many applications.
The spinning mechanism itself was found to be similar to conventional spinning of staple fibres. Conventional staple fibres such as wool and cotton are of finite length but are assembled into continuous yarns by the use of twist. The structural mechanics of such yarns is complicated, but study shows that fibre structures generated during spinning are able to convert some of the tensile load into a normal force between the fibres that in turn generates the frictional force that holds the yarn together. As twist is inserted into the fibre assembly, the position of the fibres ‘migrates’ from the surface of the yarn to the centre and back to the surface to create a coherent entangled structure. All sufficiently long fibres migrate in this way and the fibre structure created converts some of the applied tensile load into a normal force between the fibres and therefore a frictional force that is able to oppose the tension.
A difficulty facing this method for producing fibre is the prodigious levels of twist required and the consequent low production speed. It is well known in the textile industry that production speed is proportional to the spinning speed and for a conventional worsted yarn with a fibre of 20 μm diameter and a mean length of about 70 mm, the threadline speed is about 20 m/min for a spinning speed of 12 000 min−1 and a twist of 600 m−1. Clearly, if the required twist for a CNT yarn is as high as 60 000 m−1, then the yarn speed will decrease to only 200 mm/min if the spinning speed remains at 12 000 min−1. The only way to increase threadline speed for pure CNT yarns is to increase the spinning speed.
It is an object of the invention to at least in part alleviate these problems, that is to counter the slow production rates of nanofibre yarns such as carbon nanotube yarns while retaining the benefits of the structure of the yarns that follow from the solid-state method of assembly.